Sewer systems originally employed manholes which were typically poured-in-place concrete, masonry or brick. Significant cost savings have been obtained through the use of manhole sections precast at a manufacturing site and transported to a job site at which the sections are assembled.
The typical precast concrete manhole comprises a base section, at least one riser section and top section usually having a grade ring mounted thereon for final and close adjustment of the manhole to the top elevation.
The base, riser and top sections are retained in place by cooperating male and female ends of the interfitted sections cooperatively forming joints which yield a continuous, sturdy and uniform manhole. However, in order to assure watertightness of the assembled manhole, gaskets are normally provided in each joint. In vertically aligned sections it is not possible to place a gasket between the mating load bearing surfaces. Thus, present day sections have been provided with an O-ring gasket which is seated within a groove in the cylindrical surface of a male end and is compressed by the surrounding cylindrical surface. Fabrication of the groove which receives the gasket typically leads to cracking and weakening of the male end significantly reducing, if not destroying, the effectiveness of the watertight seal. Also, the seal does not employ the available gravitational forces to ensure the formation of a joint having a good watertight seal.